Standard Communication with Network Elements (SCONE) Protocol
draft-ietf-scone-protocol-04
| Document | Type | Active Internet-Draft (scone WG) | |
|---|---|---|---|
| Authors | Martin Thomson , Christian Huitema , Kazuho Oku , Matt Joras , Marcus Ihlar | ||
| Last updated | 2026-01-22 (Latest revision 2025-12-14) | ||
| Replaces | draft-thoji-scone-trone-protocol, draft-thoji-scone-protocol | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-scone-protocol-04
SCONE M. Thomson
Internet-Draft Mozilla
Intended status: Informational C. Huitema
Expires: 17 June 2026 Private Octopus Inc.
奥 一穂 (K. Oku)
Fastly
M. Joras
Meta
M. Ihlar
Ericsson
14 December 2025
Standard Communication with Network Elements (SCONE) Protocol
draft-ietf-scone-protocol-04
Abstract
This document describes a protocol where on-path network elements can
give endpoints their perspective on what the maximum achievable
throughput might be for QUIC flows.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://ietf-wg-
scone.github.io/scone/draft-ietf-scone-protocol.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-scone-protocol/.
Discussion of this document takes place on the SCONE Working Group
mailing list (mailto:scone@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/scone/. Subscribe at
https://www.ietf.org/mailman/listinfo/scone/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-scone/scone.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 June 2026.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Independent of Congestion Signals . . . . . . . . . . . . 5
3.2. Unspecified Scope . . . . . . . . . . . . . . . . . . . . 6
3.3. Per-Flow Signal . . . . . . . . . . . . . . . . . . . . . 6
3.4. Undirectional Signal . . . . . . . . . . . . . . . . . . 7
3.5. Advisory Signal . . . . . . . . . . . . . . . . . . . . . 7
3.6. Following Advice . . . . . . . . . . . . . . . . . . . . 7
4. Conventions and Definitions . . . . . . . . . . . . . . . . . 8
5. SCONE Packet . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Rate Signals . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Monitoring Period . . . . . . . . . . . . . . . . . . . . 11
5.3. Endpoint Processing of SCONE Packets . . . . . . . . . . 11
5.4. Following Throughput Advice . . . . . . . . . . . . . . . 12
6. Negotiating SCONE . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Indicating Support on New Flows . . . . . . . . . . . . . 13
6.2. Limitations of Indication . . . . . . . . . . . . . . . . 13
6.3. Indications for Migrated Flows . . . . . . . . . . . . . 14
7. Network Deployment . . . . . . . . . . . . . . . . . . . . . 14
7.1. Applying Throughput Advice Signals . . . . . . . . . . . 14
7.2. Monitoring Flows . . . . . . . . . . . . . . . . . . . . 15
7.3. Flows That Exceed Throughput Advice . . . . . . . . . . . 16
8. Version Interaction . . . . . . . . . . . . . . . . . . . . . 17
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8.1. Providing Opportunities to Apply Throughput Advice
Signals . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2. Feedback To Sender About Signals . . . . . . . . . . . . 18
8.3. Interactions with Congestion Control . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9.1. Fake SCONE Packets . . . . . . . . . . . . . . . . . . . 20
9.2. Damage to Other Protocols . . . . . . . . . . . . . . . . 21
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
10.1. Passive Attacks . . . . . . . . . . . . . . . . . . . . 22
10.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 22
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
11.1. SCONE Versions . . . . . . . . . . . . . . . . . . . . . 23
11.2. scone_supported Transport Parameter . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . 24
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Many networks have known, concrete rate limits, or apply these limits
by policy to constrain data rates. This is often done without any
indication to applications. The result can be that application
performance is degraded, as the manner in which rate limits are
enforced can be incompatible with the rate estimation or congestion
control algorithms used at endpoints.
Having the network indicate what its rate limiting policy is, in a
way that is accessible to endpoints, allows applications to use this
information when adapting their send rate.
The Standard Communication with Network Elements (SCONE) protocol is
negotiated by QUIC endpoints. SCONE provides a means for networks to
signal the maximum available sustained throughput, or throughput
advice, associated with the flows of UDP datagrams that QUIC
exchanges.
Any network function that is able to update the content of UDP
datagrams qualifies as a network element that can participate in
SCONE and provide throughput advice to QUIC endpoints.
Networks with rate limiting policies can use SCONE to send throughput
advice to cooperating endpoints to limit overall network usage.
Where congestion control signals -- such as ECN, delays and loss --
operate on a time scale of a round trip time, throughput advice
operates over a much longer period.
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This has benefits in some networks as endpoints can adapt network
usage to better suit network conditions. For example, radio networks
and battery-powered devices perform better with short, bursty
exchanges, rather than constant transmission at a fixed rate.
For endpoints, SCONE throughput advice makes network policies
visible, which can reduce wasteful probing beyond those limits.
2. Overview
QUIC endpoints can negotiate the use of SCONE by including a
transport parameter (Section 6) in the QUIC handshake. Endpoints
then occasionally coalesce a SCONE packet with ordinary QUIC packets
that they send.
Network elements that have rate limiting policies can detect flows
that include SCONE packets. The network element can indicate a
maximum sustained throughput by modifying the SCONE packet as it
transits the network element.
The propagation of SCONE packets, including the throughput advice
that is added, is shown in Figure 1.
+--------+ +---------+ +----------+
| QUIC | | Network | | QUIC |
| Sender | | Element | | Receiver |
+---+----+ +----+----+ +----+-----+
| | |
+--- SCONE --->| SCONE+advice |
| +QUIC +---- +QUIC --->|
| | | Validate QUIC packet
| | | and record rate
| | |
Figure 1: Propagation of SCONE signal
QUIC endpoints that receive modified SCONE packets observe the
indicated version, process the QUIC packet, and then record the
indicated rate.
Throughput advice only apply to the direction and path for which they
are received. A connection that migrates or uses multipath [QUIC-MP]
cannot assume that throughput advice from one path apply to new
paths. Advice for the client-to-server direction and the server-to-
client direction of each path are independent, and are expected to be
different, for reasons including asymmetric link capacity and path
diversity. Applications may choose to utilize SCONE in either or
both direction(s) of each path as they see fit.
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3. Applicability
This protocol only works for flows that use the SCONE packet
(Section 5).
The protocol requires that packets are modified as they transit a
network element, which provides endpoints strong evidence that the
network element has the power to apply a rate limiting policy; though
see Section 9 for potential limitations on this.
The throughput advice signal that this protocol carries is
independent of congestion signals, limited to a single path and UDP
packet flow, unidirectional, and strictly advisory.
3.1. Independent of Congestion Signals
Throughput advice signals are not a substitute for congestion
feedback or congestion control. Congestion signals, such as
acknowledgments or ECN markings [ECN][WHY-ECN], provide information
on loss and delay that indicate the real-time condition of a network
path. Congestion signals might indicate a throughput limit that is
different from the signaled throughput advice.
Endpoints cannot assume that the rate indicated in throughput advice
is achievable if congestion signals indicate otherwise. Congestion
could be experienced at a different point on the network path than
the network element that signals throughput advice. Therefore,
endpoints need to respect the send rate constraints that are set by a
congestion controller.
Networks can use SCONE to communicate throughput advice for reasons
other than rate limiting policies. For example, a network element in
an access network could provide throughput advice to guide
application use of network capacity, in any way that is separate from
any signals that are intended to influence congestion response.
In addition to rate limiting policies, throughput advice can indicate
temporary increases in available capacity or temporarily reduced
capacity. This includes persistent overuse, equipment faults, or
other transient issues. Providing advice is applicable if increases
or reductions are expected to last for more than one monitoring
period; see Section 5.2.
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3.2. Unspecified Scope
Modifying a packet does not prove that the throughput that is
indicated would be achievable. A signal that is sent for a specific
flow is likely enforced at a different scope. The extent of that
scope is not carried in the signal.
For instance, policy limits might apply at a network subscription
level, such that multiple flows receive the same signal, but usage
all contributes to a shared policy limit.
Endpoints can therefore be more confident in the throughput signal as
an indication of the maximum achievable throughput than as any
indication of expected throughput. In addition to endpoints
respecting congestion signals (see Section 3.1), networks might need
to monitor and enforce policies, even where applications attempt to
follow advice (see Section 7.3).
The advised throughput will likely only be achievable when the
application is the only user of throughput within the scope that the
advice applies to. In the presence of other flows, congestion limits
are likely to determine actual throughput.
This implies that signals can most usefully be applied to a downlink
flow in access networks, close to an endpoint. In that case,
capacity is less likely to be split between multiple active flows.
3.3. Per-Flow Signal
The same UDP address tuple might be used for multiple QUIC
connections. A single signal might be lost or only reach a single
application endpoint. Network elements can apply SCONE advice to all
QUIC connections that include SCONE packets to ensure that advice is
received by all application endpoints.
The signaled advice applies to the flow of packets on the same UDP
address tuple for the duration of the current monitoring period,
unless it is updated earlier or the flow ends; see Section 5.2 for
details on the monitoring period.
Rate limiting policies often apply on the level of a device or
subscription, but endpoints cannot assume that this is the case. A
separate signal can be sent for each flow.
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3.4. Undirectional Signal
Throughput advice is signaled with SCONE packets that are transmitted
as part of the flow that the advice applies to. Carrying signals in
the affected flow, in the same way that ECN signals are conveyed,
ensures that there is no ambiguity about what flow is affected.
However, this means that the endpoint that receives throughput advice
is not the endpoint that might need to adapt its sending behavior.
A receiving endpoint might need to communicate the value it receives
to the sending peer in order to ensure that the limit is respected.
This document does not define how that signaling occurs as this is
specific to the application in use.
3.5. Advisory Signal
Throughput advice indicates what one part of the network expects to
be achievable for flows that transit that portion of the network. It
is possible that very different throughput is achievable -- either
higher or lower than the advice -- as determined by congestion
control. Endpoints that receive this signal therefore need to treat
the information as advisory.
The fact that an endpoint requests throughput advice does not
necessarily mean that it will adhere to them; in some cases, the
endpoint cannot. For example, a flow may initially be used to serve
video chunks, with the client selecting appropriate chunks based on
received advice, but later switch to a bulk download for which
bitrate adaptation that cannot be similarly controlled. Composite
flows from multiple applications, such as tunneled flows, might only
have a subset of the involved applications that are capable of
handling SCONE signals. Therefore, when a network element detects
that throughput exceeds the advertised throughput advice, it might
switch to applying its policies for non-SCONE flows, using congestion
control signals.
Network conditions and rate-limit policies can change in ways that
make previously signaled advice obsolete. For example, routing
changes can cause a flow to move to a different network path. There
are no guarantees that updated advice will be sent at such events.
3.6. Following Advice
The SCONE throughput advice is advisory (see Section 3.5).
Applications that chose to follow it will do so in the way that best
suits their needs.
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The most obvious way to keep within the limits set by throughput
advice is to inform the sending peer of the limit so that the peer
can do whatever rate limiting is necessary. Alternatively, a
receiver can control the release of flow control credit (see
Section 4 of [QUIC]) to indirectly limit the sending rate of a peer.
Some applications offer options for rate control that can offer
superior outcomes. Most video applications, especially real-time and
streaming video applications, can adapt their use of network
bandwidth. For instance, typical HTTP Live Streaming [HLS] or DASH
[DASH] clients are provided with manifests that allow them to adjust
the bitrate and quality of media segments based on available network
capacity.
4. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [BCP14] when, and only when, they appear in all capitals, as
shown here.
5. SCONE Packet
A SCONE packet is a QUIC long header packet that follows the QUIC
invariants; see Section 5.1 of [INVARIANTS].
Figure 2 shows the format of the SCONE packet using the conventions
from Section 4 of [INVARIANTS].
SCONE Packet {
Header Form (1) = 1,
Reserved (1),
Rate Signal High Bits (6),
Version (32) = 0x6f7dc0fd or 0xef7dc0fd,
Destination Connection ID Length (8),
Destination Connection ID (0..2040),
Source Connection ID Length (8),
Source Connection ID (0..2040),
}
Figure 2: SCONE Packet Format
The most significant bit (0x80) of the packet indicates that this is
a QUIC long header packet. The next bit (0x40) is reserved and can
be set according to [QUIC-BIT].
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The Rate Signal High Bits field consists of the low six bits (0x3f)
of the first byte. Together with the most significant bit of the
Version field, this forms the 7-bit Rate Signal. Values for the Rate
Signal are described in Section 5.1.
The Version field contains either 0x6f7dc0fd or 0xef7dc0fd. The only
difference between these two values is the most significant bit,
which also contributes to the Rate Signal. All other bits are
identical, which facilitates detection and modification of SCONE
packets.
This packet includes a Destination Connection ID field that is set to
the same value as other packets in the same datagram; see
Section 12.2 of [QUIC].
The Source Connection ID field is set to match the Source Connection
ID field of any packet that follows. If the next packet in the
datagram does not have a Source Connection ID field, which is the
case for packets with a short header (Section 5.2 of [INVARIANTS]),
the Source Connection ID field is empty.
SCONE packets MUST be included as the first packet in a datagram.
This is primarily to simplify the process of updating throughput
advice in network elements. This is also necessary in many cases for
QUIC versions 1 and 2 because packets with a short header cannot
precede any other packets.
5.1. Rate Signals
A Rate Signal is a 7-bit unsigned integer (0-127). The high six bits
are the Rate Signal High Bits, and the least significant bit is the
most significant bit of the Version field.
When sent by a QUIC endpoint, the Rate Signal is set to 127.
Receiving a value of 127 indicates that throughput advice is unknown,
either because network elements on the path are not providing advice
or they do not support SCONE. All other values (0 through 126)
represent the ceiling of rates advised by the network element(s) on
the path.
Throughput advice follows a logarithmic scale defined as:
* Base rate (b_min) = 100 Kbps
* Bitrate at value n = b_min * 10^(n/20)
where n is an integer between 0 and 126 represented by the Rate
Signal.
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Table 1 lists some of the potential values for signals and the
corresponding bitrate for each.
+============+=============+
| Bitrate | Rate Signal |
+============+=============+
| 100 Kbps | 0 |
+------------+-------------+
| 112 Kbps | 1 |
+------------+-------------+
| 126 Kbps | 2 |
+------------+-------------+
| 141 Kbps | 3 |
+------------+-------------+
| 1 Mbps | 20 |
+------------+-------------+
| 1.12 Mbps | 21 |
+------------+-------------+
| 10 Mbps | 40 |
+------------+-------------+
| 11.2 Mbps | 41 |
+------------+-------------+
| 100 Mbps | 60 |
+------------+-------------+
| 112 Mbps | 61 |
+------------+-------------+
| 1 Gbps | 80 |
+------------+-------------+
| 1.12 Gbps | 81 |
+------------+-------------+
| 10 Gbps | 100 |
+------------+-------------+
| 11.2 Gbps | 101 |
+------------+-------------+
| 100 Gbps | 120 |
+------------+-------------+
| 112 Gbps | 121 |
+------------+-------------+
| 199.5 Gbps | 126 |
+------------+-------------+
| Unknown | 127 |
+------------+-------------+
Table 1: Examples of
SCONE signals and
corresponding rates
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5.2. Monitoring Period
The time over which throughput advice applies is defined to be a
period of 67 seconds.
Protocol participants can use a different period, depending on their
role. Senders can limit their send rate over any time period up to
67 seconds. Network elements can monitor and apply limits to send
rates using time period of at least 67 seconds.
The choice of 67 seconds is a compromise between competing interests.
Longer periods allow applications more flexibility in terms of how to
allocate bandwidth over time. Shorter periods allow networks to
administer policies more tightly. Also, when circumstances change,
applications are better able recover without exceeding limits for a
significant time if they have exceeded limits.
The choice of 67 seconds, as a prime number, also helps avoid
synchronization with other periodic effects that are commonly
measured in whole seconds. This includes segment length or key frame
intervals in video applications, but also includes timers for
middleboxes; see Section 4.3 of [RFC4787]. Any repeating phenomenon
at a 67 second interval is therefore unlikely to be due to other
periodic effects.
5.3. Endpoint Processing of SCONE Packets
Processing a SCONE packet involves reading the value from the Rate
Signal field. However, this value MUST NOT be used unless another
packet from the same datagram is successfully processed. Therefore,
a SCONE packet always needs to be coalesced with other QUIC packets.
A SCONE packet is defined by the use of the long header bit (0x80 in
the first byte) and the SCONE protocol version (0x6f7dc0fd or
0xef7dc0fd in the next four bytes). The 7-bit Rate Signal can be
extracted by combining the low 6 bits of the first byte with the most
significant bit of the version field. A SCONE packet MAY be
discarded, along with any packets that come after it in the same
datagram, if the Source Connection ID is not consistent with those
coalesced packets, as specified in Section 5.
A SCONE packet is discarded if the rate signal is unknown (127).
A SCONE packet MUST be discarded if the Destination Connection ID
does not match one recognized by the receiving endpoint.
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If a connection uses multiple DSCP markings [RFC2474], the throughput
advice that is received on datagrams with one marking might not apply
to datagrams that have different markings.
5.4. Following Throughput Advice
Endpoints that receive throughput advice can advise their peer of the
limit so that the peer might limit the amount of data it sends over
any monitoring period (Section 5.2). Alternatively, the endpoint
might change its own behavior to effect a similar outcome indirectly,
which might use flow control or changes to request patterns.
An endpoint that receives throughput advice might receive multiple
different values. If advice is applied by applications, applications
MUST apply the lowest throughput advice received during any
monitoring period; see Section 5.2.
After a monitoring period (Section 5.2) without receiving throughput
advice, any previous advice expires. Endpoints can remove any
constraints placed on throughput based on receiving throughput
advice. This does not mean that there are no limits, either in
policy or due to network conditions, only that these limits are now
unknown. Other constraints on usage will still apply, which
necessarily includes congestion control and might include other,
application-specific constraints.
The relatively long duration of the monitoring period means that this
is preferable to disabling sending completely. The cost is that loss
of information about recent send rate might result in temporarily
exceeding the rates indicated by throughput advice. In comparison,
applications retain throughput usage state when throughput advice
increases.
This approach ensures that network elements are able to reduce the
frequency with which they send updated signals to as low as once per
monitoring period. However, applying signals at a low frequency
risks throughput advice being reset if no SCONE packet is available
for applying signals (Section 7.1), or the rewritten packets are
lost. Sending the signal multiple times increases the likelihood
that the signal is received.
6. Negotiating SCONE
A QUIC endpoint indicates that it is able to receive SCONE packets by
including the scone_supported transport parameter (0x219e).
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Each endpoint independently indicates willingness to receive SCONE
packets. An endpoint that does not include the scone_supported
transport parameter can send SCONE packets if their peer includes the
transport parameter.
The scone_supported transport parameter MUST be empty. Receiving a
non-zero length scone_supported transport parameter MUST be treated
as a connection error of type TRANSPORT_PARAMETER_ERROR; see
Section 20.1 of [QUIC].
This transport parameter is valid for QUIC versions 1 [QUIC] and 2
[QUICv2] and any other version that recognizes the versions,
transport parameters, and frame types registries established in
Sections 22.2, 22.3, and 22.4 of [QUIC].
6.1. Indicating Support on New Flows
All new flows that are initiated by a client that supports SCONE MUST
include bytes with values 0xc8 and 0x13 as the last two bytes of
datagrams that commence a new flow if the protocol permits it. This
indication MUST be sent in every datagram until the client receives
any datagram from the server, at which point the client can be
confident that the indication was received.
A client that uses a QUIC version that includes length-delimited
packets, which includes QUIC versions 1 [QUIC] and 2 [QUICv2], can
include an indicator of SCONE support at the end of datagrams that
start a flow. The handshakes of these protocols ensures that the
indication can be included in every datagram the client sends until
it receives a response -- of any kind -- from the server.
6.2. Limitations of Indication
This indication does not mean that SCONE signals will be respected,
only that the client is able to negotiate SCONE. A server might not
support SCONE and either endpoint might choose not to send SCONE
packets. Finally, applications might be unable to apply throughput
advice or choose to ignore it.
This indication being just two bytes means that there is a non-
negligible risk of collision with other protocols or even QUIC usage
without SCONE indications. This means that the indication alone is
not sufficient to indicate that a flow is QUIC with the potential for
SCONE support.
Despite these limitations, having an indication might allow network
elements to change their starting posture with respect to their
enforcement of their rate limit policies.
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6.3. Indications for Migrated Flows
Applications MAY decide to indicate support for SCONE on new flows,
including when migrating to a new path (see Section 9 of [QUIC]). In
QUIC version 1 and 2, the two byte indicator cannot be used.
Sending a SCONE packet for the first few packets on a new path gives
network elements on that path the ability to recognize the flow as
being able to receive throughput advice and also gives the network
element an opportunity to provide that throughput advice.
To enable this indication, even if an endpoint would not otherwise
send SCONE packets, endpoints can send a SCONE packet any time they
send a QUIC PATH_CHALLENGE or PATH_RESPONSE frame. This applies to
both client and server endpoints, but only if the peer has sent the
transport parameter; see Section 6.
7. Network Deployment
QUIC endpoints can enable the use of the SCONE protocol by sending
SCONE packets Section 5. Network elements can then use SCONE and
replace the Rate Signal field (Section 7.1) according to their
policies.
7.1. Applying Throughput Advice Signals
A network element detects a SCONE packet by observing that a packet
has a QUIC long header and one of the SCONE protocol versions
(0x6f7dc0fd or 0xef7dc0fd).
A network element then conditionally replaces the most significant
bit of the Version field and the Rate Signal High Bits field with
values of its choosing.
A network element might receive a packet that already includes a rate
signal. The network element replaces the rate signal if it wishes to
signal a lower value for throughput advice; otherwise, the original
values are retained, preserving the signal from the network element
with the lower policy.
The following pseudocode indicates how a network element might detect
a SCONE packet and replace the existing rate signal (packet_signal)
with a new rate signal (target_signal) that encodes the throughput
advice of this network element.
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is_long = packet[0] & 0x80 == 0x80
packet_version = ntohl(packet[1..5])
if is_long and (packet_version & 0x7fffffff) == SCONE_VERSION_BITS:
packet_signal = ((packet[0] & 0x3f) << 1) | (packet_version >> 31)
if target_signal < packet_signal:
packet[0] = (packet[0] & 0xc0) | (target_signal >> 1)
packet[1] = (packet[1] & 0x7f) | (target_signal << 7)
Once the throughput advice signal is updated, the network element
updates the UDP checksum for the datagram.
To avoid throughput advice expiring, a network element needs to
ensure that it sends updated rate signals with no more than a
monitoring period (Section 5.2) between each update. Because this
depends on the availability of SCONE packets and packet loss can
cause signals to be missed, network elements might need to update
more often. Ideally, network elements update advice in SCONE packets
at least twice per monitoring period, to match endpoint behavior (see
Section 8.1).
At the start of a flow, network elements are encouraged to update the
rate signal of the first few SCONE packets it observes so that
endpoints can obtain throughput advice early.
Senders that send a SCONE packet or network elements that update
SCONE packets every 20–30 seconds is likely sufficient to ensure that
throughput advice is not lost. To reduce the risk of synchronization
across multiple senders, which may cause network elements to miss
updates, senders can include a small random delay.
A network element MUST NOT alter datagrams to add SCONE packets or
synthesize datagrams that contain SCONE packets. The latter will not
be accepted and the former, even if they do not exceed the path MTU
as a result, can be detected by applications and could be ignored.
This document does not define a mechanism to support detection, but
one might be added in future.
7.2. Monitoring Flows
Sending throughput advice is optional for any network. A network
that sends throughput advice might, also optionally, choose to
monitor flows to determine whether applications are following advice.
This section outlines a method that a network element could use to
determine whether advice is being followed. Network deployments that
choose to monitor are free to follow any monitoring regime that suits
their needs.
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This monitoring algorithm is guidance only. However, monitoring any
more strictly than the following could mean that an application might
be incorrectly classified as not following advice. A looser
monitoring approach, such as monitoring over a longer time window
than the monitoring period (67s) or using a higher rate than is
signaled, has no risk of incorrect classification.
When a network changes the value it intends to signal, applications
need time to adjust their sending behavior. As a result, any
monitoring needs to allow time for SCONE packets to be updated, for
those packets to be received by endpoints, and for applications to
adapt.
A network element can then monitor affected flows to determine
whether the throughput advice it provided was followed.
A network element SHOULD base its monitoring on the maximum value
that was configured to apply during the preceding two monitoring
periods. If the network element cannot update the throughput advice
in every SCONE packet (or can do so only infrequently), a longer
period might be used to account for the possibility that the updated
SCONE packets are lost. This allows applications time to receive
advice and adapt their sending rate.
Any monitoring and policy enforcement could be implemented in
different network elements than the ones that signal throughput
advice. However, network elements MUST NOT enforce throughput based
on throughput advice found in SCONE packets received from other
entities, because unlike endpoints, network elements do not have the
capability to validate other QUIC packets contained in the same
datagram; see Section 9.1.
7.3. Flows That Exceed Throughput Advice
A network could deploy policy enforcement that drops or delays
packets to ensure that applications do not exceed throughput limits
set in policy.
SCONE allows networks to provide advice to applications, so that
stricter limits, which can be inefficient and lead to worse
application performance, are not necessary in all cases.
Some applications will not support SCONE. Other applications either
will not or cannot follow throughput advice.
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Networks can monitor flows to determine if applications follow
advice; see Section 7.2. A network could choose to either disable or
loosen policy enforcement for flows where SCONE is active, but re-
enable or tighten enforcement if monitoring indicates that throughput
advice is not being respected.
8. Version Interaction
The SCONE protocol defines two versions (0x6f7dc0fd and 0xef7dc0fd)
that cover different ranges of bitrates. This design allows for:
* Support for both very low bitrates (down to 100 Kbps) and very
high bitrates (up to 199.5 Gbps)
* Graceful handling of network elements that might only recognize
one version.
8.1. Providing Opportunities to Apply Throughput Advice Signals
Endpoints that wish to offer network elements the option to add
throughput advice signals can send SCONE packets at any time. This
is a decision that a sender makes when constructing datagrams.
When sending SCONE packets, endpoints MUST include the SCONE packet
as the first packet in a datagram, coalesced with additional packets.
Upon confirmation that the peer is willing to receive SCONE packets,
an endpoint SHOULD include SCONE packets in the first few UDP
datagrams that it sends. Doing so increases the likelihood of
eliciting early throughput advice from network elements, allowing
applications to apply that advice from the early stages of the data
transfer.
After that, endpoints that seek to receive throughput advice on a
flow MUST send a SCONE packet at least twice each monitoring period;
see Section 5.2.
Sending SCONE packets more often might be necessary to:
Avoid missing advice: If SCONE packets are not sent, updated, and
received for an entire monitoring period, an application might
incorrectly assume that no advice is being provided.
Reduce latency: The time between SCONE packets determines the
maximum delay between changes in throughput advice and when that
advice can be received and acted upon.
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A sender can track the receipt of the coalesced QUIC packet and send
another SCONE packet when loss is detected. However, it is likely
simpler to send SCONE packets more often.
Sending a SCONE packet every 20–30 seconds is likely sufficient to
ensure that throughput advice is not lost, though endpoints might
send a packet every few seconds to improve responsiveness. This
period could be determined by how quickly an application is able to
respond to a change in throughput advice.
For example, a streaming application that fetches video segments that
are 5 seconds in length might send SCONE packets on a similar
cadence. A real-time conferencing application might send more often.
In either case, the length of the monitoring period (Section 5.2)
limits how fast any application can react.
Though sending SCONE packets more than once each round trip time
might help reduce exposure to packet loss, it is better to spread
updates over time rather than to send multiple SCONE packets in less
frequent bursts.
The main cost associated with sending SCONE packets is the reduction
in available space in datagrams for application data.
A network element that wishes to signal updated throughput advice
waits for the next SCONE packet in the desired direction; see
Section 7.1.
8.2. Feedback To Sender About Signals
Information about throughout advice is intended for the sending
application. Any signal from network elements can be propagated to
the receiving application using an implementation-defined mechanism.
This document does not define a means for indicating what was
received. That is, the expectation is that any signal is propagated
to the application for handling, not handled automatically by the
transport layer. How a receiving application communicates the
throughput advice signal to a sending application will depend on the
application in use.
Different applications can choose different approaches. For example,
in an application where a receiver drives rate adaptation, it might
not be necessary to define additional signaling.
A sender can use any acknowledgment mechanism provided by the QUIC
version in use to learn whether datagrams containing SCONE packets
were likely received. This might help inform whether to send
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additional SCONE packets in the event that a datagram is lost.
However, rather than relying on transport signals, an application
might be better able to indicate what has been received and
processed.
SCONE packets could be stripped from datagrams in the network, which
cannot be reliably detected. This could result in a sender falsely
believing that no network element applied a throughput advice signal.
Senders will therefore proceed as though there was no advice.
8.3. Interactions with Congestion Control
SCONE and congestion control both provide the application with
estimates of a path capacity. They are complementary. Congestion
control algorithms are typically designed to quickly detect and react
to congestion, i.e., to the "minimum" capacity of a path. SCONE
informs the endpoint of the maximum capacity of a path based on
network rate limit policy, network conditions, or a combination of
the two.
Consider for example a path in which the bottleneck router implements
some form of Early Congestion Notification [ECN]. If the path
capacity diminishes, queues will build up and the router will
immediately start increasing the rate at which packets are marked as
"Congestion Experienced". The receiving endpoint will notice these
marks, and inform its peer. The incoming congestion will be detected
in 1 round trip time (RTT). This scenario will play out whatever the
reason for the change in capacity, whether due to increased
competition between multiple applications or, for example, to a
change in capacity of a wireless channel.
9. Security Considerations
The modification of packets provides endpoints proof that a network
element is in a position to drop datagrams and could apply a rate
limit policy. Section 8.1 states that endpoints only accept signals
if the datagram contains a packet that it accepts to prevent an off-
path attacker from inserting spurious throughput advice signals.
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Some off-path attackers may be able to both observe traffic and
inject packets. Attackers with such capabilities could observe
packets sent by an endpoint, create datagrams coalescing an arbitrary
SCONE packet and the observed packet, and send these datagrams such
that they arrive at the peer endpoint before the original packet.
Spoofed packets that seek to advertise a higher limit than might
otherwise be permitted also need to bypass any rate limiters. The
attacker will thus get arbitrary SCONE packets accepted by the peer,
with the result being that the endpoint receives a false or
misleading rate limit.
The recipient of a throughput advice signal therefore cannot
guarantee that the signal was generated by an on-path network
element. However, the capabilities required of an off-path attacker
are substantially similar to those of on path elements.
The actual value of the throughput advice signal is not
authenticated. Any signal might be incorrectly set in order to
encourage endpoints to behave in ways that are not in their
interests. Endpoints are free to ignore limits that they think are
incorrect. The congestion controller employed by a sender provides
real-time information about the rate at which the network path is
delivering data.
Similarly, if there is a strong need to ensure that throughput advice
is respected, network elements cannot assume that the signaled advice
will be respected by endpoints.
9.1. Fake SCONE Packets
Attackers that can inject packets could compose arbitrary "SCONE-
like" packets by selecting a pair of IP addresses and ports, an
arbitrary rate signal, a valid SCONE version number, an arbitrary
"destination connection ID", and an arbitrary "source connection ID".
A coalesced "1RTT" packet will start with a plausible first octet,
and continue with the selected destination connection ID followed by
a sufficiently long series of random bytes, mimicking the content of
an encrypted packet.
Endpoints will reject such packets because they do not contain valid
QUIC packets, but network elements cannot detect this. All the
network elements between the injection point and the destination will
have to process these packets.
Attackers could send a high volume of these "fake" SCONE packets in a
denial of service (DOS) attempt against network elements. The attack
will force the intermediaries to process the fake packets. If
network elements are keeping state for ongoing SCONE flows, this
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might exhaust memory resources. The mitigation is the same as for
other distributed DOS attacks: limit the rate of SCONE packets that a
network element is willing to process; possibly, implement logic to
distinguish valid SCONE packets from fake packets; or, use generic
protection against Distributed DOS attacks.
Attackers could also try to craft the fake SCONE packets in ways that
trigger a processing error at network elements. For example, they
might pick connection identifiers of arbitrary length. Network
elements can mitigate these attacks with implementations that fully
conforms to the specification of Section 5.
9.2. Damage to Other Protocols
Network elements that update SCONE packet fields might do that for
datagrams exchanged in other protocols. This could result in damage
to those protocols.
The most serious damage occurs when every datagram is modified,
because that could mean that the protocol is effectively unable to
operate end-to-end.
To that end, network elements MUST only update the content of
datagrams on a given address tuple a few times each monitoring
period. Network elements MAY update more often immediately after a
change in their throughput advice, to reduce the reaction time from
senders.
In addition, some heuristics might be used to detect SCONE-compatible
QUIC flows. This includes identification of a QUIC handshake on the
flow, the presence of indications (Section 6.1), or other heuristics.
If these heuristics indicate a non-QUIC flow, the safest option is
for network elements to disable updating of datagrams.
10. Privacy Considerations
The focus of this analysis is the extent to which observing SCONE
packets could be used to gain information about endpoints. This
might be leaking details of how applications using QUIC operate or
leaks of endpoint identity when using additional privacy protection,
such as a VPN.
Any network element that can observe the content of that packet can
read the throughput advice that was applied. Any signal is visible
on the path, from the point at which it is applied to the point at
which it is consumed at an endpoint. On path elements can also alter
the SCONE signal to try trigger specific reactions and gain further
knowledge.
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In the general case of a client connected to a server through the
Internet, we believe that SCONE does not provide much advantage to
attackers. The identities of the clients and servers are already
visible through their IP addresses. Traffic analysis tools already
provide more information than the throughput advice set by SCONE.
There are two avenues of attack that require more analysis:
* that the passive observation of SCONE packets might help identify
or distinguish endpoints; and
* that active manipulation of SCONE signals might help reveal the
identity of endpoints that are otherwise hidden behind VPNs or
proxies.
10.1. Passive Attacks
If only few clients and server pairs negotiate the usage of SCONE,
the occasional observation of SCONE packets will "stick out". That
observation, could be combined with observation of timing and volume
of traffic to help identify the endpoint or categorize the
application that they are using.
A variation of this issue occurs if SCONE is widely implemented, but
only used in some specific circumstances. In that case, observation
of SCONE packets reveals information about the state of the endpoint.
If multiple servers are accessed through the same front facing
server, Encrypted Client Hello (ECH) may be used to prevent outside
parties to identify which specific server a client is using.
However, if only a few of these servers use SCONE, any SCONE packets
will help identify which specific server a client is using.
This issue will be mitigated if SCONE becomes widely implemented, and
if the usage of SCONE is not limited to the type of applications that
make active use of the signal.
QUIC implementations are therefore encouraged to make the feature
available unconditionally. Endpoints might send SCONE packets
whenever a peer can accept them.
10.2. Active Attacks
Suppose a configuration in which multiple clients use a VPN or proxy
service to access the same server. The attacker sees the IP
addresses in the packets behind VPN and proxy and also between the
users and the VPN, but it does not know which VPN address corresponds
to what user address.
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Suppose now that the attacker selects a flow on the link between the
VPN/proxy and server. The attacker applies throughput advice signals
to SCONE packets in that flow. The attacker chooses a bandwidth that
is lower than the "natural" bandwidth of the connection. A reduction
in the rate of flows between client and VPN/proxy might allow the
attacker to link the altered flow to the client.
+--------+
| Client |------.
+--------+ \ +-------+
'---->| | +--------+
+--------+ | VPN |<==========>| |
| Client |------------->| / |<==========>| Server |
+--------+ | Proxy |<==========>| |
.---->| | ^ +--------+
+--------+ / +-------+ |
| Client |======' |
+--------+ ^ Apply throughput advice signal
\
\
Observe change
An attacker that can manipulate SCONE headers can also simulate
congestion signals by dropping packets or by setting the ECN CE bit.
That will also likely result in changes in the congestion response by
the affected client.
A VPN or proxy could defend against this style of attack by removing
SCONE (and ECN) signals. There are few reasons to provide per-flow
throughput advice signals in that situation. Endpoints might also
either disable this feature or ignore any signals when they are aware
of the use of a VPN or proxy.
11. IANA Considerations
This document registers new QUIC versions (Section 11.1) and a QUIC
transport parameter (Section 11.2).
11.1. SCONE Versions
This document registers the following entries to the "QUIC Versions"
registry maintained at https://www.iana.org/assignments/quic
(https://www.iana.org/assignments/quic), following the guidance from
Section 22.2 of [QUIC].
Value: 0x6f7dc0fd
Status: permanent
Specification: This document
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Change Controller: IETF (iesg@ietf.org)
Contact: QUIC Working Group (quic@ietf.org)
Notes: SCONE Protocol - Low Range
Value: 0xef7dc0fd
Status: permanent
Specification: This document
Change Controller: IETF (iesg@ietf.org)
Contact: QUIC Working Group (quic@ietf.org)
Notes: SCONE Protocol - High Range
11.2. scone_supported Transport Parameter
This document registers the scone_supported transport parameter in
the "QUIC Transport Parameters" registry maintained at
https://www.iana.org/assignments/quic
(https://www.iana.org/assignments/quic), following the guidance from
Section 22.3 of [QUIC].
Value: 0x219e
Parameter Name: scone_supported
Status: Permanent
Specification: This document
Date: This date
Change Controller: IETF (iesg@ietf.org)
Contact: QUIC Working Group (quic@ietf.org)
Notes: (none)
12. References
12.1. Normative References
[BCP14] Best Current Practice 14,
<https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/rfc/rfc8999>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[QUIC-BIT] Thomson, M., "Greasing the QUIC Bit", RFC 9287,
DOI 10.17487/RFC9287, August 2022,
<https://www.rfc-editor.org/rfc/rfc9287>.
[QUICv2] Duke, M., "QUIC Version 2", RFC 9369,
DOI 10.17487/RFC9369, May 2023,
<https://www.rfc-editor.org/rfc/rfc9369>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/rfc/rfc2474>.
12.2. Informative References
[DASH] "Information technology — Dynamic adaptive streaming over
HTTP (DASH) — Part 1: Media presentation description and
segment formats", ISO/IEC 23009-1:2022, August 2022,
<https://www.iso.org/standard/83314.html>.
[ECN] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/rfc/rfc3168>.
[HLS] Pantos, R., Ed. and W. May, "HTTP Live Streaming",
RFC 8216, DOI 10.17487/RFC8216, August 2017,
<https://www.rfc-editor.org/rfc/rfc8216>.
[QUIC-MP] Liu, Y., Ma, Y., De Coninck, Q., Bonaventure, O., Huitema,
C., and M. Kühlewind, "Managing multiple paths for a QUIC
connection", Work in Progress, Internet-Draft, draft-ietf-
quic-multipath-18, 8 December 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
multipath-18>.
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[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <https://www.rfc-editor.org/rfc/rfc4787>.
[WHY-ECN] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/rfc/rfc8087>.
Acknowledgments
Jana Iyengar made significant contributions to the original TRAIN
specification that forms the basis for a large part of this document.
The following people also contributed significantly to the
development of the protocol: Alan Frindell, Gorry Fairhurst, Kevin
Smith, and Zaheduzzaman Sarker.
Authors' Addresses
Martin Thomson
Mozilla
Email: mt@lowentropy.net
Christian Huitema
Private Octopus Inc.
Email: huitema@huitema.net
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Additional contact information:
奥 一穂
Fastly
Matt Joras
Meta
Email: matt.joras@gmail.com
Marcus Ihlar
Ericsson
Email: marcus.ihlar@ericsson.com
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